X-ray tube



rye.

Julle 25, 1929. A, BOUWERS AL X-RAY TUBE Filed Feb. 9, 1924 Patented June 25, 1929.

UNITED STATES PATENT oFFlcE. l

ALBERT :Bouwen-s AND WILLEM HENDRIK VAN es SANDEBAKHUY'ZEN, orn1Nrrlio` VEN, NETHERLANDS, AssIGNoRs To NAAMLoozE VENNOOTSCHAP PHILIPS" e-LOEILAMPENFABRIEHEN, or EINDHOVEN, NETHERLANDS.

X-RAY TUBE. i

Application filed February 9', 1924, Serial No. 691,658, and in the Netherlands September 28, 1923.

The present invention relates to improvements 1n incandescent cathode X-ray tubes. .Known tubes of this type are for example those constructed by Wehneltand Trenkle 5 (Fii'tzungsberichte der physikalisch medizinischen Soziett, Erlangen 37,312 (1905)) and those constructed by Coolidge. i

In contradistinction to the X-ray tube of lllelflnelt and Trenlle in which the vacuum 1U was not extremely high, the vacuum `in the X-ray tube of Coolidge according to his de- `scriptiomis to be as high as possible in order to avoid ionization. For this latter purpose the gas pressure in Coolidges tubes is about .l 0.00005 millinietres ot mercury, the upper limit of pressure being fixed at 0.0006 millimetres, thus much lower than the gas pressure in X-ray tubes the operation of which is based on gas ionization and in Which the gas I nessure varies between 0.001 and 0.01 millimetres of mercury. In order to ensure as far` as possible the removal of the very last gas residues it has even been proposed to introduce into the X-ray tubes or' the Coolidge or zirconium, which uponheating, will combine with the said gas residues. n y

lt has been found that the objectionable infiuence of. gas ionization can be suppressed in another Way than by exhausting the X-ray tube to a high vacuum and it ispossible to obtain very satisfactory incandescent cathode X-ray tubes with a properly chosen gaseous filling.

Incandescent cathode X-ray tubes according to the present invention are characterized by a gaseous filling of hydrogen, helium, or a mixture ot the said gases, said filling having a pressure such that no appreciable gas ionization occurs and the production `of positive y ions is practically eliminated.

As a rule the gaseous pressure Will be chosen higher than 0.0006 millimetres of 1ner cury., Very good results have been obtained with X-ray tubes in which during operation, the field at the anticathode Was ver intensive. The more intensive the fieldy anticathode in such X-raytubes, the higher is the gas pressure at which gas ionizationl begins to have an appreciable influence.

Consequently the inventionl also comprises type suitable substances, for example thorium i at the Probably ionization due principally to` secondary electrons generated at the anticathode. Therefore by providing a large `field at thefiocal spot, the said seconda-ry electrons will be drawn back to the anticathodebeiore having an opportunity of ionizing molecules ofthe gaseous filling. A A y ln ,order to ensure an intensive field at the anticathode,the X-ray tube may be soproportioned that the'distance between the cathode and the anticathode is very small. The Xsray tube may alsobe so constructed that when connected to a high voltage circuit, the field is quite intensively concentrated before the anticathode and the entire potential difi terence between the cathode and anticathode is substantially before the anticathode and even very close before it. n .i y

X-ray tubes according to the invention will satisfy the condition thatfon supplying the high voltage between the cold cathode and the anticathode, no current Will pass through "the tube. In case such a current should flow, pl1enomena of ionization might be produced during the operation of the tube with the incan descent cathode, which phenomena Would have a detrimental effect on the o )eration of the X-ray tube. It vvill be possile to prevent such phenomena of ionization by taking care that the distances between all parts-between which a high difference of potential eX ists are very small and by constructing the tube in such a manner that all lines of torce pass over a minimum distance in the interior of the X-ray tube. It Will be essential" in that case also that those parts of the X-rayl` tube, surrounded by Walls of insulating material, Vshould be so constructed and proportioned that the lines of force strike the said walls as far as possible in a normal or perpendicular direction and that the distance between said Wallsis small. i `The present invention also comprises an incandescent cathode X-ray tube having a gaseous filling of hydrogen, helium,uor a 1`niX- ture of said gases, the cathode heilig arranged" l.and ythe portion ofthe lsurface of the anticathode to be struck by theV cathode rays Will be placed in oradjacent t-he said aperture.

X-ray tubes according to thev invention have the advantage vthat they are better i adapted thanhigh vacuum X-ray tubes to be operated with alternating current Without the fanticathode being heated by the bombardment of electrons to sucha temperature r that a counter-current isproduced.

` 'Way' of'exrample'two constructions of X-ray Moreover the cooling of' the anticathode in X-ray rtubes according to the present invenltion Will be assisted by t-he heatsconduction of the gaseous -filling. YParticularly when hydrogen is used-as a gaseousV filling, the anticathode Vwill reach'a less high temperature duri-ngthe operation of the tube than has heretofore been the case in the usual high vacuum Xfray tubes and thus theprobability d of the' anticathode being burnt or pitted by the intensively concentrated beamV of' cathode rays Will be very small.

. The y'nve ntion will be more clearly understood by reference to the accompanying drawings, in which Figures 1 and 2 illustrate by tubes embodying the invention.

"Figure l'. shows an X-ray tube having a bulb 1,van incandescent cathode 2 and an anticathode 3.y The incandescent cathode may be of,r tungsten andis connected to leading-in Wires 4, 4 sealed in the bulb. rlhe cathode 2 is surrounded byla focussing device 5 for cathode rays; sa1d` device being 1n electrical conductlve connection to the cathode and supy ported by supportingwires 6, 6. The said focussing'` device comprises a metal cylinder to which apmeta'l hemispherical element is fixed. 1 The antic'athode may be of tungsten fixedin copper which -may be sealed in the I" bulb by means of platinum. l `It is so arranged thatthe cathode rays can strike the antica-thode only over part of its surface. As there is no objection to al relatively high gas pressurev Within theX-ray tube according to A -ythe present invention, it is not necessary that the air enclosed Within the tube be removed by means of a high vacuum pump; a properly operating oil-pump is quite sufiic'ient for the object of theinvention. `While using such pump, the tube 'should be continuously Washed With the gas lwith which the X-ray tube isto be filled.

Furthermore it is necessary that the lass of the bulb and the anti-cathmlc be fPreed from gases as the gases they generally contain, might injuriously influence the opera tion of the X-ray tube. As soon as the objectionable gases have as far as possible been removed by the continuously flowing gas which is intended to fill the X-ray tube, the gaseous filling is given the desired pressure. By operating the X-ray tube thereupon f or some time some gas may disappear from the tube, thereby reducing the gas pressure. ly subsequently supplying gas to the tube and repeating this operation several times, the desired gas pressure can be restored, and thus a stat-e of affairs can be ensured in which the gas pressure no longer varies objectionably. lVhen the'said state of affairs is reached, the X-ray tube is sealed olf from the pump.

It is possible to obtain excellent results with a tube of the construction described, in Which the distance from cathode to anticathode is only 0.8 centmetros, so that when connected to a circuit supplying 100,000 volts a very intensive field is obtained before (he anticathode, the tube having a hydrogen lill ing under a pressure of 0.01 millimetres of mercury. The current passing through the, tube remains perfectly constant during a radiographic exposure of several minutes and even after several hundreds of exposures no impairment of the good properties of the X- raytube is perceptible. The gas pressure of 0.01 milliinetres used in this tube is even higher than is the practice in )Cray tubes the operation of which is based on gas ionization. According to the present inven tion it is however, possible to supply X-ray tubes With a gaseous filling of a materially higher pressure than 0.01 millimetres of mercury, for instance one millimetro. ln any event,'the gas pressure will be above 0.0000 milli-nietres of mercury, and lies preferably between 0.0006 and 0.02, within which range a pressure of from 0.001 to 0.01 generally yields the best results.

Figure 2 shows an X-ray tube with a met al envelope. In this highly suitable construction, the incandescent cathode 0 is arranged Within a metal vessel 7 to the wall of which is fixed a metal cover S provided with an aperture before which part of the surface of theanticathode 10 is located. Opposite. the said anticathode, the metal \f'ossel is provided With a Window for the passage of the X-rays generated. The said window comprises a glass cap 11 hermetically sealed to the metal vessel at 19, and in addition an annular metal plate 12 for example of iron is provided screening the X-rays and having the leadingin Wires for the incam'lescent cathmle fixed to it. The X-ray tube is filled with hy drogen or helium of a pressure liietween 0.01 and 1 millimetres of mercury. One leadingin Wire 13 for the incandescent cathode passes `through the metal plate 12 from which it is insulated and is sealed in the glass cap l1. The other leading-in wire is consti tuted by the conductive connection o t the terminal lll, the metal plate l2 and tlfiecmetal envelope 7. The cathode is made incandescent by the passage of an electric current supplied by a battery 17 with which aregulating resistance 18 is connected in series. A metal cylinder 1G disposed before the weld l5 is sealed to the metal envelope 7 and serves to protect the weld l5 against bombarding electrons and thus against electrostatic charges. The distance from the anticathode `to the .metal envelope 7, the metal cover 8 and the metal cylinder 1G so small throughout that at the gas pressure used there is no risk ol' a discharge occurring between the said metal parts when the tube is connected to a source ot high voltage.

The cathode rays emitted by the cathode can strike only a small ar-ea ot the anticathode, owing to the particular shape of wall and cover of the metal vessel and to the arrangement ol the antieathode in relation to the said cover. In the case oi. the cover being provided with an aperture having a diameter oil 20 millimetres, the focal spot is found to have a diameter of some 2 millimetres only.

lVhcn supplying a` high voltage between the cathode and anticathode the whole iield between the cathode and anticathode will be concentrated in a small space before the anticathrale, as the metal vessel 7 has a potential which is approximately equal to or lower than that of the cathode and an Xsray tube according to the present invention and constructed described is found to operate in an eX- cellent manner.

lit is obvious that in addition to the constructions hereinbetore described there are many other constructions of Xafay tubes which iiall within the scope of the invention. Moreover it the geometric dimensions of the `ifi-ray tube arc so chosen that the field before the anticathode is concentrated to a higher extent than in the construction shown in Figure 2, it is possible to use higher pressures than l millimetre of mercury.

ln many cases it may be desirable to introduce a drying-medium for example into the bulb olf the X-ray tube.

Vilhile in the appended claims we refer speciiically to a hydrogen illing, we desire it to be understood that ttor the purposes of our present invention helium, or a mixture of helium and hydrogen, are to be considered equivalents oil hydrogen, and that such claims are to be interpreted accordingly.

What we claim is:

l. An X-ray tube comprising an envelope, cooperating electrodes therein one olf which has connections for heating it independently of the other, the space within said envelope being provided with a gaseous filling oi at least one of the two `elements oli' smallest atomic number having a pressure higher than 0.0006 mm. of mercury suoli that there does not occur appreciable ionization by collision during the operation of the tube.

2. In an X-ray tube operating by substantially pure electron conduction independently oit positive gas ionization, the combination ot a cathode, an anode and an inelosing envelope provided with a gaseous illing of at least one of the two elements of' smallest atomic number having a pressure between 0.0006 and 0.02 mm. of mercury.

3. An incandescent cathode X-ray tube containing a gaseous filling of at least one oit the two elements of smallest atomic number having a pressure higher than 0.0006 min. of mercury such that no appreciable gas ionization occurs, and including an incandescent cathode and an anticathode, the distance between the anticathode and the incandescent cathode being very small.

4L. An incandescent cathode X-ray tube containing a gaseous lilling oit at least one of the two elements of smallest atomic number having' a pressure higher than 0.0006 mm. oi: mercury such that no appreciable gas ionization occurs, and including a metal vessel having a wall which forms part of the outer envelope ot the tube, an incandescent cathode mounted within the metal vessel, and an anticathode separate from said vessel by an insulation capable of resisting the operating voltage to be supplied between the incandesn cent cathode and the anticathode.

5. An .incandescent cathode X-ray tube containing a gaseous filling oi? at least one of the two elements oi smallest atomic number having a pressure higher than 0.0006 mm. oit mercury such that the hydrogen is not appreciably ionized, and including a metal vessel having a wall which terms part of the outer envelope of the tube, a cathode mounted within the metal vessel, and an anticathode separated. from said vessel by an insulation capable ol resisting the operating voltage to be supplied between the cathode and the anticathode, said metal vessel being locally constrieted to form a small aperture for focusing the electrons passing therethrough and a portion of the surface of the anticathode designed to be struck by the cathode rays being placed adjacent the said aperture, whereby the cathode rays can strike only a small area of the anticathode.

6. An Xiay tube comprising an evacuated envelope, an anticathode and a double terminal cathode located in said envelope, and a gaseous filling of at least one of the two elements of smallest atomic number in said envelope under a pressure greater than 0.0000 mm. of mercury.

7. An X-ray tube oi the incandescent cathode type, having an envelope the outer Wall of which is partly of metal, and compris-l electrically connected with the incandescent ing a gaseous filling of at least one of the two cathode being very small. l0 elements of smallest atomic number under a. In testimony whereof We aiix oui signapressure higher than 0;()006 mm. of mercury tures, at the city of Eindhoven, this 19th day sueh that no appreciable gas ionization will of January, 1924.

occur, an incandescent cathode, and an anticathode in thertube, the distance between the ALBERT BUWERS- anticathode and parts of the tube which are WILLEM HENDRIK vl de SANDE BAKHUYZEN- 

